Polyvinyl alcohol (PVOH) is a water soluble, biodegradable resin with excellent aroma and oxygen barrier properties and resistance to most organic solvents. Other desirable attributes of PVOH include anti-static properties, grease and oil resistance and heat sealability. The polymer is used extensively in adhesive, textile sizing and paper coatings. However, the end uses of PVOH have been limited despite its excellent chemical, mechanical and physical properties to those uses in which it is supplied as a solution in water. This limitation is partly due to the fact that vinyl alcohol polymers in the unplasticized state have a high degree of crystallinity and show little or no thermoplasticity before the occurrence of decomposition which starts at about 170.degree. C. and becomes pronounced at 200.degree. C., which is below its crystalline melting point.
U.S. Pat. No. 5,051,222 discloses a method for making thermoplastic (melt extrudable) PVOH compositions which comprises providing sufficient energy to a PVOH which is at least partially crystalline to both melt the PVOH and substantially eliminate the crystallinity in the PVOH melt while simultaneously removing energy from the PVOH melt at a rate sufficient to avoid decomposition of the PVOH. The melt is then extruded, preferably into a strand, rapidly cooled and cut into pellets for subsequent thermoprocessing into the desired product or article.
As a result of the availability of such thermoplastic PVOH compositions, the desirable physical and mechanical properties of PVOH combined with thermoplasticity have opened new market opportunities for PVOH, especially in packaging applications where the contents must be dissolved or dispersed in water. Examples of such packaging applications for these PVOH materials include pesticides which are applied as a water spray, caustic cleaners or detergents which are dissolved during use, and process chemicals such as pigments, dyes or carbon black which are dissolved or dispersed in water. The advantages of using a water soluble film for packaging the above materials include the following:
elimination of human exposure to highly toxic or concentrated chemicals; PA0 improvement in measuring accuracy; and PA0 elimination of the need to clean and discard toxic chemical containers after use. PA0 a) 5 to 95 wt % PVOH, PA0 b) 5 to 95 wt % thermoplastic PEO; and PA0 c) 1 to 30 wt % plasticizer, based on PVOH.
While there are water soluble PVOH films available for packaging these materials, they typically have at least one deficiency. First, the commercially available PVOH films lack the alkaline stability necessary for packaging caustic cleaning products. Normally, the partially hydrolyzed PVOH resin is used to produce a cold water soluble film for packaging alkaline products. However, the partially hydrolyzed PVOH in contact with alkaline materials will undergo further hydrolysis reaction to produce a fully hydrolyzed PVOH film. This fully hydrolyzed PVOH film becomes cold water insoluble due to an increase in the overall degree of hydrolysis. In addition, the fully hydrolyzed PVOH film is highly crystalline. This crystallinity causes the film to be more brittle and susceptible to cracking at low humidity which may result in an accidental spill of the chemicals if the package is broken.
A second drawback of PVOH resin is salt solution insolubility. The PVOH resin has a slower rate of solubility in salt water containing low levels of mono- or divalent salts and will precipitate out of solution at salt concentration over 10 wt %. This shortcoming limits the end use of PVOH packaging additives that need to be delivered to brackish media, as in the case of additives for oil drilling or related industries.
Many attempts have been reported in the literature to overcome some, but not all of the above mentioned shortcomings. U.S. Pat. Nos. 3,441,547; 3,505,303 and 4,747,976 discuss modifications or copolymerization of PVOH to produce a pH stable product. Other approaches have been through the blending of PVOH with other water soluble polymers to overcome the shortcomings of PVOH resins. These approaches are disclosed in U.S. Pat. Nos. 2,850,741; 3,695,989; 4,692,494 and 4,481,326.